Primary ignition vs crankshaft position

The purpose of this test is to examine both crank sensor and ignition primary voltage as a possible cause of engine misfire at high RPM.

How to perform the test

View connection guidance notes.

1. Connect a 10:1 attenuator to PicoScope Channel A.  
2. Connect Channel A to the negative terminal on the ignition coil and to earth.
3. Use manufacturer data to identify the output/signal circuit for the crank sensor.
4. Connect PicoScope Channel B to the crank sensor output/signal circuit.
5. Start the engine and allow it to idle.
6. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
7. Start the scope to see live data.
8. Increase engine RPM until high RPM misfire occurs.
9. With your waveforms on screen stop the scope.
10. Stop the engine and turn off the ignition.
11. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Note 

This helpfile refers to a 10:1 attenuator. If you are using a 20:1 attenuator please adjust the Probe settings for the relevant channel. These settings can be found under the Channel Options button, then: Probe > 20:1 Attenuator.

Example waveform

Waveform notes

In this waveform we can observe the output voltage from the Crank Angle Sensor (shown in red) at the same time as monitoring the Ignition's Primary trace (shown in blue). The main reason for evaluating these two waveforms together is to identify the cause of any impending misfires at higher engine revs.

The picture shows the 'missing tooth' reference point and the primary induced voltage. The offset between these two points will vary between different vehicle manufacturers as the 'missing tooth' is not always in the same position.

As the engine speed is increased, the distance between the reference point and the induced voltage alters due to the engine's ignition timing advance. The gap in the red trace is due to the 'missing tooth' in the flywheel or reluctor and is used as a reference for the Electronic Control Module (ECM) to ascertain the engine's position. Some systems use one reference point per revolution while others use two. The trace from the crank sensor should maintain a constant voltage at a given engine speed, while the primary ignition trace shows the firing of the ignition circuit.

Should the engine start to misfire at speed, ensure that the crank sensor signal is not breaking down: this may be seen as an intermittent trace or as a reduction in amplitude. If the CAS output remains constant, the primary picture may falter: this could be due to either a fault coil or amplifier.

Further guidance

GT390